The present invention relates to cam mechanisms and in particular, although not exclusively, to cam mechanisms used to control the inlet and exhaust valves of an internal combustion engine.
Modern high performance internal combustion engines have been developed to give a maximum power output at high engine speeds. In order to achieve this, the profile of the cam controlling opening and closing of the valves is designed to give high lift with long duration, in order to encourage gas flow at high speeds.
With such designs, the gas flow at low engine speeds is very much compromised. Under such conditions, incoming air is spilled back into the manifold due to late closing of the inlet valve, producing a corresponding reduction in the torque output available at low speeds. Also, the exhaust gas is released too early, reducing the expansion ratio of the engine and hence its efficiency. Furthermore, the overlap period where both inlet and exhaust valves are open is too large and allows free flow of air and fuel through the exhaust valve, thus causing emission problems.
The present invention provides a cam mechanism which may be adjusted in accordance with the engine speed, to vary the duration of opening of the valve.